Transcript
Page 1: Design of lower limb exoskeleton

Design of Exoskeleton for Paraplegic Patients

Alok Bharadwaj | Aditya S. N | Anirudh V KaushikR V College of Engineering

Bangalore

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Overview

• Kinematics• Dynamics• Design• Analysis• Controlling• Design Features – Comfort level,– Manufacturability, – Compactness and structural stability.

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Kinematics of Mechanism

1200

1200

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Kinematics of Mechanism

• Hip (Passive joint)

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Kinematics of Mechanism

• Hip and Knee joint• Use of mechanical

stoppers to limit the angular movement of limbs.

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Kinematics of Mechanism

• Foot joint (Ball and Socket)

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Dynamics of Mechanism

Assumptions

Mass of all links = 4kgMass of all motors = 4kgMass of battery pack = 2 kg Weight of human considered = 1000N

Leg portion Length Mass Weight

L1 450mm 5kg 50N

L2 500mm 3kg 30N

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Dynamics of Mechanism

• Case 1 - Standing up

Figure A Figure B

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Dynamics of Mechanism

• Standing up Torque required by motor at M2

= (1000 N) X (450 mm) = 450000 Nmm

However, as the motion begins, the position of upper body keeps changing. Thus, due to change of Center of gravity, required torque is much lower progressively.

Also, the crutches take up most of the body weight as the person can easily bend upper body to stand up.

This value is rounded off to 50000 Nmm shared by each leg. Thus, torque at each motor M2 is 25 Nm.

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Dynamics of Mechanism

• Case 2 - Climbing stairsTorque required by knee motor= (1100 N) X (335 mm)=368500 Nmm

After considering the load taken up by using crutches and bending forward in order to climb stairs, this value is considerably lesser. (Assumed to be 50 Nm for each knee motor in each leg)

Thus, the maximum torque at knee considering previous case too becomes 50 Nm.

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Dynamics of Mechanism

• Case 3 – Raising leg (maximum extent)

Torque required by motor at hip= (110 N) X (450 mm)=49500 Nmm

Thus, the maximum torque required at the hip joint is around 50 Nm at each motor of the hip joint.

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Design

• Frame

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Design

• Thigh Link

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Design

• Shin Link

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Design

• Leg

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Design

• Complete Assembly – Ez Walk

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Design

• Draft

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Design

• Choosing the Battery:– For given maximum load @ 20rpm, the torque

required is 50Nm.– Required power = 100W per motor at 24V DC– Total max power input = 400W for motors– Overall power output = 500W – Battery assumed to last for two hours– Capacity needed 40000mAh– Weight of LiPo batteries ~ 3.5Kg

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Analysis

• Material Used: Al 6061– Ultimate strength: 310Mpa– Yield strength: 276Mpa– Shear strength: 207Mpa– FOS considered: 3

• Failure mechanism: Von Mises

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Analysis

• Critical Link: Thigh link

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Analysis

• Critical section

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Controlling the mechanism

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Initiating walking

• Angle measurement using strain gauge measures the angle of upper body with respect to ground.

• Length of step proportional to step length.

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Use of Ultrasonic Sensors

• 2 sensors are used in each leg – one at hip strap and one at foot

• Hip sensor – senses variation in the terrain

• Foot sensor – senses the vertical distance to the ground

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Different cases requiring control

Walking

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Different cases requiring control

Climbing Stairs UP

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Different cases requiring control

Climbing Stairs DOWN

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Safety Feature

• Crutches are provided to give proper balance when the person bends forward

• While climbing down, if the person leans forward (variation in strain gauge), the mechanism is turned off preventing his fall.

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Advantages of Design

Comfort• Use of straps for even distribution of load• Weight evenly distributed on both sides• Weight of mechanism not being transferred to

the body.

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Advantages of Design

Manufacturability• Links used and Aluminum 6061 are readily available

in the market.• Assembly is simple. Most assembly is just through

press fit. Fasteners are required only at a few places.

• No requirement of a CNC for manufacturing of any component

• Ball socket joints can be readily bought off the shelf

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Advantages of Design

Weight• Aluminum has a very low density making the

mechanism very light• Evenly distributed loads at all locations. No point

loads.• Weight evenly distributed b/w both legs resulting

in no lateral CG offset. This makes the mechanism easy to use

• Low weight motors

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Advantages of Design

Special features• Adjustable links• Portability – ease of assembly. Links can be

press fitted into each other. Very less use of fasteners.


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